Hydro-geochemical controls on removal of Cr(VI) from contaminated groundwater by anion exchange

Column experiments using strongly basic anion exchange resin under acidic conditions of pH (3.6–5) and conducted with groundwater sources having strong acidic anions (Cl− and image) in excess of weak acidic anions (image and image) but with markedly different Cr(VI) concentrations demonstrate that the anion chemistry of groundwater controls the efficacy of the ion exchange process for removing dissolved oxyanions of Cr(VI). Although the proportions of image relative to image in the influent waters are reduced due to acidification, image also exists when total Cr(VI) in the solution is high. Breakthrough of Cr occurs in the ion exchange column gradually and earlier than the expected breakthrough time calculated on the basis of flow characteristics and exchange capacity of the resin. The influent with higher Cr concentration (e.g., 95 mg/L) shows a square wave shaped breakthrough curve resembling an ideal type of breakthrough. The diffused nature in the early part of breakthrough is more pronounced with the influent with lower concentration of Cr (e.g., 3.3 mg/L). Thus, the breakthrough pattern with high concentration of Cr in the influent does not follow the prediction made in some previous studies describing an advection–reaction model of chromatographic flow, coupled with empirical reaction parameters obtained from chromate–chloride and chromate–sulfate isotherms experimentally determined in the range of aqueous Cr(VI) < 20 mg/L. One-dimensional advective–reactive–dispersive modeling with a full account of all the major anion exchanges, namely, image, image, image, image, image, and image explains the effects of interfering anions on the Cr breakthrough. Available separation factors are used as first-order approximations of the equilibrium constants of the exchange reactions. If multi-component exchange is not considered the model shows that the Cr breakthrough is nearly ideal and the exchange capacity of the exchanger can be utilized to its full potential. When multi-component exchanges in the column are taken into consideration, then the model shows breakthrough patterns that closely match those observed in the experiments. Thus, the early and gradual breakthrough of Cr(VI) oxyanions during anion exchange cannot be wholly attributed to the unique characteristics of the chromate–chloride isotherms. The overall hydro-geochemistry of a site must be considered when evaluating the applicability of ion exchange to remediation of Cr-contaminated groundwater.